In recent years, progress of electronic devices has been remarkable, and reduction in size and weight of portable electronic devices has been in progress at a rapid pace. Accordingly, batteries to be used as their power sources are also required to have a high energy density so that they can be made small in size and light in weight. Particularly, as a non-aqueous secondary battery, attention has been drawn to a non-aqueous secondary battery employing lithium or a lithium alloy for a negative electrode and using an electrolyte-impregnated solid electrolyte, or a lithium ion secondary battery employing a carbonaceous material for a negative electrode.
A binder for a battery is usually employed in the form of a binder composition having a polymer to be a binder dissolved or dispersed in water or in an organic solvent. In such a binder composition, an electrode active material is dispersed, and applied to the surface of the current collector, and then the solvent is evaporated to fix the electrode active material to the electrode surface. If the binder fails to fix an adequate amount of the electrode active material to the electrode, a battery having a large initial capacity cannot be obtained, or the electrode active material falls off from the electrode by repetition of charge and discharge, whereby the capacity of the battery decreases.
As the binder composition, two types i.e. an organic solvent type binder composition and an aqueous binder composition are available. As an organic solvent type binder composition, one having polyvinylidene fluoride dissolved in an organic solvent such as N-methylpyrrolidone is widely used (e.g. Patent Document 1). However, if charge and discharge are repeated by a battery having incorporated an electrode obtained by using such an organic solvent type binder composition, there is a problem such that the electrode active material fixed to the electrode is likely to fall off. Further, such an electrode has a problem that it is poor in flexibility, and cracking is likely to be formed on the surface of the electrode when an external force is exerted e.g. in a take-up step in the process for producing a battery.
On the other hand, as an aqueous binder composition, one having carboxymethyl cellulose or the like added as a thickener to an aqueous dispersion of styrene/butadiene copolymer rubber latex produced by using water as a dispersing medium by an emulsion polymerization method employing a surfactant, has been used (e.g. Patent Document 2). Such a rubber polymer is excellent in flexibility as compared with the polyvinylidene fluoride, but the polymer is poor in oxidation resistance, particularly in charge and discharge durability at a high temperature.
In order to solve such problems, as a rubbery fluorinated copolymer, a copolymer comprising repeating units derived, respectively, from vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene, has been proposed as a polymer for a binder composition for an electrode (e.g. Patent Document 3). However, such a fluorinated copolymer tends to undergo swelling at from 60 to 80° C. in an electrolyte of a usual non-aqueous battery using, as a solvent, propylene carbonate, 2-methyltetrahydrofuran, γ-butyrolactone or the like, and it is likely to bring about a decrease in the battery capacity during discharge.